In air conditioning a space, a major objective is to establish a design temperature and humidity for that space. Although usually the design condition will vary over a small tolerance range, sometimes the design conditions are determined by the requirements of mechanical, electrical or electronic equipment, for example, a photographic laboratory or an operating theatre in a hospital.
To maintain design conditions, maximum sensible and latent heat loads are determined, and a supply of air, usually partly recirculated and partly fresh, is passed over a relatively cold heat exchanger surface of a dehumidifier, where it is cooled and moisture is condensed in order to offset the sensible and latent heat loads.
Removal of humidity (latent heat) from air by chemical means is already known and this forms no part of the invention. For many reasons dehumidification by passing air over a low temperature extended heat exchange surface is preferred, but throughout the full range of air inlet temperature from 4.degree. C. to 45.degree. C. and the outlet temperature from -2.degree. C. to 44.degree. C., and the range of inlet moisture content from 0.004 to 0.022 and the outlet humidity ratio from 0.004 to 0.021 (kg. of moisture per kg of dry air), effective air conditioning has not been achieved without (in many instances) overcooling the air in order to offset the humidity load. This is because under present practice it has not been practicable to obtain from the air conditioning system a performance which achieves one of the major aims of an air conditioning system, that is a coil condition curve which is compatible with the load ratio line. In the methods which have been adopted heretofore it is necessary (in many instances) to overcool to sufficiently dehumidify the air to offset the latent heat loads, resulting in the outlet end of the coil condition curve being at a dry bulb temperature which is less than the required temperature at the inlet condition of the load ratio line. When over-cooling occurs, reheating is frequently required to rectify these conditions. Since the coefficient of performance of an air conditioner is usually greater than unity (sometimes as much as five), the energy consumed in reheating can be a large proportion of the total energy consumed by the system.
Historically, the factors governing air conditioning system design have been built on numerous approximations some of which are well founded although their effect is minor, and others have resulted in over design, waste of energy and poor performance. However, an important factor in an air conditioning system is its heat exchanger (or dehumidifier) which functions to both cool and dehumidify air in order to reduce both the sensible and latent heat of the air being cooled (that is, the specific enthalpy).
By making reference to early practice, it is possible to observe the effects of the invention. Early practice has mis-interpreted the statement that "the higher the air velocity, the higher the bypass factor". Though this as an isolated statement can be shown to be true, it is not true when qualified by the constraints that are imposed by the principles of air conditioning in system design. Taken in total complex of an air conditioning system design, it is shown hereunder that the opposite is true.
The term "bypass factor", and another approximate term used synonomously with bypass factor (because they are nearly equal), "wet bulb depression ratio", relate in the context of this specification, to describe per unit mass of flow of dry air, that is, the degree of dehumidification relative to sensible cooling that will be expected. A high bypass factor (or a high wet bulb depression ratio), under the constraints that apply to air conditioning problems, is associated with a low face velocity. The statement, "the higher the air velocity, the higher the bypass factor", fails to apply to the air conditioning system situation because the higher air velocity of necessity requires a dehumidifier to have a smaller face and free flow area in order to maintain the desired mass flow rate of air that is relevant to an air conditioning system requirement, and to the associated temperature difference across the dehumidifier. The air conditioning application requires qualifications to the statement. To reduce the face area would reduce the size of the dehumidifier. Even though high velocities are associated with improved total heat exchange performance, there would be insufficient heat exchange surface unless the dehumidifier makes up part of the loss of size in face area by some increase in depth, or in a change of design of coil, such as increased number of fins per unit length. This is due to the constraint that dehumidifiers in an air conditioning application must be selected for a particular mass flow rate (or volume flow rate) of air, and a deeper dehumidifier, due to the higher face velocity, will have a reduced bypass factor (not an increased bypass factor). This is established by the equation: EQU Bypass factor=(Bypass factor per one row deep).sup.n
where n represents the rows in depth.
Since the bypass factor is a number less than 1 and n is a positive exponent, the bypass factor will reduce on increase of rows in depth of a dehumidifier.
The effect of even a minor increase in depth is far greater than an increase in air velocity. Thus in the context of air conditioning application an increase in air face velocity will reduce the bypass factor.
The following example illustrates the very important and opposite conclusion when applied to an air conditioning system having a fixed mass flow of air:
Reference is made to the third edition of the authoritative textbook "Modern Air Conditioning, Heating and Ventilating" by Carrier, W. H.; Cherne, R. E.; Grant W. A. and Roberts, W. H., published by Pittman Publishing Co., New York, U.S.A. On page 319, the following statement appears:
"The bypass factor decreases . . . as air velocity decreases".
The following table is also found on that page:
TABLE 1 ______________________________________ TYPICAL BYPASS FACTORS FOR COOLING SURFACE ______________________________________ (5/8 in. OD tube, 8 crimped helical fins per inch, 0.008 in. thick, 13/32 in. fin height, surface ratio 12.3) Face velocity (fpm) Rows 300 400 500 600 Deep Bypass factor ______________________________________ 1 0.61 0.63 0.65 0.67 2 0.38 0.40 0.42 0.43 3 0.23 0.25 0.27 0.29 4 0.14 0.16 0.18 0.20 5 0.09 0.10 0.11 0.12 6 0.05 0.06 0.07 0.08 7 0.03 0.04 0.05 0.06 8 0.02 0.02 0.03 0.04 ______________________________________ (5/8 in. OD tube, 14.4 smooth helical fins per inch, 0.012 in. thick at base, 13/32 in. fin height, surface ratio 21.5) 1 0.48 0.52 0.56 0.59 2 0.23 0.27 0.31 0.35 3 0.11 0.14 0.18 0.20 4 0.05 0.07 0.10 0.12 5 0.03 0.04 0.06 0.07 6 0.01 0.02 0.03 0.04 ______________________________________
Contrary to the teaching of that textbook, this invention is based partly on the discovery that the bypass factor increases as air velocity decreases when the constraints imposed by an air conditioning system are imposed.
The Table above indicates that a four row deep coil with a 600 foot per minute face velocity has a bypass factor of 0.20 and the same coil at 300 feet per minute face velocity has a lower bypass factor of 0.14. The above comparison however is involving two mass flow (or volume flow) terms. In the context of selection for an air conditioning system, two different unrelated problems are being compared since a designer must select a dehumidifier based on a particular mass flow of air that fits the problem which in turn is associated with a particular temperature difference across the load ratio line.
For a fixed mass flow of air, the 600 fpm coil should have half the face area in order to maintain the comparison of the same mass flow of air. In such a comparison even though the higher velocity coil would have a greater capacity for heat transfer it would be necessary to use a deeper dehumidifier. It it is desired to compare the two coils under the conditions that they have equal total heat exchange surface then the halved 600 feet per minute face area dehumidifier would have twice the depth or 8 rows. It is known that the bypass factor reduces with coil depth. It is shown herein that the rate of increase of bypass factor with increase of air velocity is small in relation to the decrease resulting from the increase of coil depth.
If Table 1 is studied to observe the relative effect, it will be seen that a 4 row deep coil at 300 feet per minute will have a bypass factor of 0.14, and at 600 feet per minute a bypass factor of 0.20. However, when considered under conditions of equal air flow rates by adjusting the face area and depth of the coil, a 5 row deep coil, at 600 feet per minute will have a bypass factor of 0.12, but with 8 rows deep, 0.04.
Thus it is seen that a lower velocity coil under conditions of constant mass flow rate (as in the case of an air conditioning application) will have the larger bypass factor. For example as shown above, 0.14 compared with 0.12 and 0.04. (For sake of complying with the Table referred to, the above are imperial units).
The main object of this invention is to provide a method of air conditioning wherein the energy requirement of the system is reduced, and a second related object is to reduce the required size of the machinery and cooling tower (even though these benefits may be gained to some extent at the expense of a larger dehumidifier cross-section).